A magnetic resonance apparatus generally includes a superconducting coil, a gradient coil, a radio-frequency (RF) coil, a computer system and other auxiliary equipments.
The superconducting coil is adapted for generating a main imaging magnetic field, and the gradient coil is adapted for proving a gradient field, which can be used to cooperate with the main imaging magnetic field to form an imaging magnetic field (referred to as B0 field for short). The RF coil includes a transmitter coil and a receiver coil. The transmitter coil is usually a body coil. The transmitter coil transmits a RF pulse to stimulate protons in human body to resonate. The receiver coil receives a magnetic resonance signal emitted from the human body. The computer system controls pulse excitation, signal sampling, data operation, image display and etc.
However, the B0 filed has disadvantages of non-uniformity and drift.
In magnetic resonance imaging, a high uniform field is beneficial for improving signal-to-noise ratio of image, ensuring an accurate spatial orientation, reducing artifacts, and improving scan view and etc.
Currently, a plurality of methods for improving uniformity of the imaging magnetic field are provided in practical applications. For example, a method for calibrating an imaging magnetic field in a magnetic resonance apparatus is provided in a Chinese patent application CN101509964C. The calibration method applies a sequence of navigation echo in data sampling, the navigation echo and an image echo are from different spatial dimensions, and a phase encoding gradient is applied only before sampling the image echo. Therefore, a two dimensional image data and a two dimensional navigation data are obtained. Based on the sampled data, an image processing is performed to calibrate the non-uniformity of the imaging magnetic field.
Besides the non-uniformity, stability is also an important factor to evaluate the imaging magnetic field. The stability of the imaging magnetic field is classified into thermal stability and time stability. The gradient coil adapted for providing the gradient field releases a lot of heat, which would result in temperature rising of the magnet warm bore. Furthermore, the temperature of the magnet warm bore may rise because of eddy currents, resulting in variation of magnetic conductivity, which will further result in drift of the imaging magnetic field and a negative effect on image quality. A method for avoiding magnetic field drift by controlling temperature variation is provided in a Chinese patent application CN101427919C.
However, the methods mentioned above either require many additional hardware to correct for anomalies in the imaging magnetic field, or correct the image in a post-acquisition process, which may result in poor quality of images and require further improvement.